Recently, with the spread of thermal heads, the demand for heat-sensitive recording media has been suddenly increased. In particular, in prepaid cards which are being rapidly spreading in the field of communications, transportations, distributions, etc., it has frequently be practiced to indicate a magnetic information on a card as a visible information. Such magnetic cards have been widely used as highway cards, prepaid cards in department stores, supermarkets, etc., JR (Japanese Railways Co.) orange cards, etc.
However, in such a recording medium, the area on which a visible information can be indicated is limited and thus, for example, in the case of a large amount of a prepaid card, it sometimes happens that when the balance is additionally indicated, the additional information cannot be indicated. In such a case, a new card is usually reissued to cope therewith, which causes a problem of increasing a cost.
To overcome such a problem, it has been investigated on a reversible recording medium capable of repeatedly recording and erasing an information on the same area. If such a recording medium is used, an old unnecessary information can be erased and a new information can be indicated, whereby it becomes unnecessary to reissue a new card due to lacking in an additional area to be indicated. Further, by using such a recording medium as, for example, a facsimile paper, resorces can be saved, which contributes to overcome the environmental problem.
Hitherto, as a heat-sensitive recording medium capable of reversibly recording and erasing an information, a recording medium having a heat-sensitive layer formed by dispersing an organic low molecular weight material such as a higher alcohol, a higher fatty acid, etc., in a resin matrix such as polyvinyl chloride, a vinyl chloride-vinyl acetate copolymer, polyester, polyamide, etc., is proposed as disclosed in JP-A-54-119377, JP-A-55-154198, and JP-A-2-1363 (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
The principle of the formation and erasion of images in such a reversible heat-sensitive recording medium is in the difference of a transparency of the heat-sensitive layer when the heat-sensitive layer is heated to a different temperature and returned to normal temperature. That is, when such a recording medium is heated to a definite temperature and returned to normal temperature, the heat-sensitive layer shows a transparent state, but when the recording medium is heated to other temperature than the above definite temperature and returned to normal temperature, the heat-sensitive layer shows a white turbid state. For recording to such a heat-sensitive recording medium, a thermal head is used and for erasing the recorded information, a heat roll, a hot stamp, a thermal head, etc., is used.
Since the white turbid state of the recording medium described above is the state of utilizing the scattering of light, the hiding property is low and the contrast is poor. To overcome the problem, it is proposed to form a metal reflective layer or a colored layer on the surface of the reversible heat-sensitive recording layer, i.e., between the recording layer and a substrate.
However, when a metal reflective layer is formed on the surface of the reversible heat-sensitive recording layer, since all the light transmitting through the transparent portion is reflected, the white turbid portion formed by printing becomes difficult to confirm, depending on a seeing angle.
When a black or blue colored layer is formed on the surface of the reversible heat-sensitive recording layer, the scattered light from the white turbid portion is less and a sufficient contrast is not obtained. As a result, it becomes necessary to increase the thickness of the reversible heat-sensitive recording layer, which results in increasing the energy required for recording, and when recording is repeated, the early deterioration of the recording layer becomes severe.
Furthermore, for improving the contrast at the white turbid portion, a method of coloring a substrate and then forming a reversible heat-sensitive recording layer on the substrate through an air layer is proposed as described in JP-A-4-220400, but in this method, there are problems that the production method is complicated and the cost is increased.